We have observed that normal mouse mammary gland, not unlike certain human and rodent mammary neoplasias, can also become refractory to hormone-dependent growth regulation; however, in contrast to mammary tumors, in normal mammary gland this refractoriness is fully reversible. Estrogen and progesterone, two major growth regulatory hormones in mammary gland, are believed to produce their biologic effects via hormone-receptor-mediated mechanisms. Recent studies of estrogen receptor ontogeny in the uterus have shown that receptors appear in the stroma before they appear in the epithelium. However, hormone exposure of the tissue at a time when receptors are present only in the stroma results in cell proliferation in the epithelium. It has been hypothesized that hormonally-induced growth factors in stromal cells are responsible for the growth of the epithelium. The specific aim of the proposed research is to determine if regulation of epithelial cell proliferation by estrogen and/or progesterone in mammary gland occurs directly in response to these hormones or whether it is mediated indirectly by the response of the stromal cells. The first step will be to determine the distribution of estrogen (ER) and progesterone receptors (PgR) in the various epithelial and stromal cell types that make up the range of mammary gland structures. Mammary glands at different developmental stages with differing responses to estrogen and progesterone will be examined: (1) early postnatal gland will be investigated to determine the ontogeny of ER and PgR in epithelial and stromal cells in relation to the ontogeny of cell proliferative responses to estrogen and progesterone; (2) adult virgin gland will be studied as a highly responsive state to the cell proliferative effects of both estrogen and progesterone; and (3) postpartum gland will be studied for potential changes in cellular distribution of ER and/or PgR that could account for the refractoriness to growth regulation that is observed during lactation. Cellular localization of ER and PgR will be determined histologically by the technique of steroid autoradiography. For all of the developmental stages in vivo cell proliferation in response to treatment with estrogen or progesterone will be quantitated, in parallel, by DNA historadiography. Such studies will provide important insights into the roles of stromal and epithelial cells in mediating and/or modifying growth regulation by estrogen and/or progesterone, the basis for refractoriness to growth regulation in mammary cancer, and the therapeutic reversal of hormone independence in human mammary cancer. (D)